This invention relates to a negative-feedback amplifier circuit comprising a plurality of transistors in so-called cascode connection in the manner known in the art. The negative-feedback amplifier circuit is for amplifying an input electric signal into an output electric signal
In cable television (CATV) systems or the like, trials have recently been made to accommodate an increased number of video signal channels as many as 100 or more. In order to cope with such a large number of video signal channels, it is required to provide a wide-band amplifier capable of amplifying a video signal over a wide band. Specifically, each video channel is generally required to have a bandwidth on the order of 6 MHz. Therefore, the wide-band amplifier is required to perform low-distortion and high-gain amplification over a wide band of at least 600 MHz. Preferably, the wide-band amplifier of the type is adapted to perform low-distortion amplification over a band as wide as 1 GHz, taking a future increase in number of channels into consideration.
For example, Japanese Unexamined Patent Publication (JP-A) No. 7-183735 (183735/1995) discloses one example of such an amplifier. This amplifier comprises a negative-feedback amplifier circuit which Includes a plurality of field effect transistors (FET's) in so-called cascode connection and a negative-feedback circuit for achieving a desired gain. Japanese Unexamined Patent Publication (JP-A) No. 3-52407 (52407/1991) discloses an amplifier comprising bipolar transistors instead of the above-mentioned FET'S.
FIG. 1 shows a conventional negative-feedback amplifier circuit of the type. The negative-feedback amplifier circuit comprises a first-stage FET 1 and a second-stage FET 2 in so-called cascade connection, and a negative-feedback circuit 3 which couples a drain of the second-stage FET 2 to a gate of the first-stage FET 1 with a resistor Rf connected therebetween to adjust a gain. The negative-feedback amplifier circuit has an input terminal IN connected to the gate of the first-stage FET 1 and an output terminal OUT connected to the drain of the second-stage FET 2.
In the negative-feedback amplifier circuit illustrated In FIG. 1, however, the gain is adjusted or changed by selecting a resistance value of the resistor Rf. In this event, impedances of the input terminal IN and the output terminal OUT are also changed. This results in impedance mismatching with external circuits connected to the input terminal IN and the output terminal OUT. This is because the gain and the Impedances can not be controlled independently from each other.